Molecular dynamics simulations can be used to compute static structure factors (S(q)) and provide an interpretation of the underlying periodic atomic ordering. Here, we present a method for classifying interatomic distances that allows for quantitative peak assignment and visualization of atoms that most contribute to each peak in calculated S(q) for soft matter. The method is illustrated for the low-wavenumber peaks (near q = 0.45, 1.4, and 3.0 Å-1) and the shoulder (near q = 0.75 Å-1)in the calculated S(q) for the ionic liquid tetradecyltrihexylphosphoniumbis(trifluoromethylsulfonyl)amide. The first sharp diffraction peak (FSDP) near 0.45 Å-1increases in height with increasing temperature, and the partial S(q) associated with polar-polar components in the wavenumber range of the FSDP also increases with increasing temperature. We find that the increase in the polar-polar partial S(q) does not correspond to an increase in polar-polar periodic atomic ordering with increasing temperature, but instead is associated with a decrease in such ordering. This was determined by decomposing the polar-polar partial S(q) into positive and negative components (each indicating periodic atomic ordering) by separately summing the Fourier transform summands used to calculate it. The positive and negative polar-polar partial S(q) components both decrease in magnitude with increasing temperature, indicating decreased polar-polar ordering. The polar-polar partial S(q) increases overall with increasing temperature due to the positive component decreasing in magnitude less than the negative component. This work highlights the utility of decomposing calculated S(q) and partial S(q) into positive and negative components. It also emphasizes the importance of simulations as a tool to help interpret static structure factors determined by X-ray or neutron scattering.

Quantitation and visualization of static structure factor peaks for a phosphonium-based ionic liquid

2019 Fall National Meeting

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